Two coloring methods are usually used for the color-changing treatment of keratin fibers. By the first method, the coloring is accomplished with oxidative or permanent colorants by use of a mixture of different developers and couplers and an oxidant. By this method, if necessary, it is possible to add a direct (non-oxidative) dye if the coloring result is to be adjusted or special coloring effects are to be achieved. The second method involves the exclusive use of direct dyes which in an appropriate carrier composition are applied to the fibers. This method is easy to apply, very gentle and causes only minor damage to the keratin fibers. The direct dyes used for this purpose are subject to many requirements. For example, they must be unobjectionable in toxicological and dermatological terms and must make it possible to attain colorations of a desired intensity which, among other things, presupposes sufficient water solubility. Moreover, the colorations obtained are required to exhibit good light stability, acid resistance and abrasion resistance.
The advantages of direct coloring over oxidative coloring lie in generally lower hair damage, because normally the former method involves working at low pH values (below 9) and without an oxidant. Direct dyes are also used in various ways as color shade-adjusting aids in oxidative colorants. For a direct (non-oxidative) colorant for keratin fibers it is usually necessary to use a combination of different non-oxidative dyes. Because the selection of dyes that can be used in colorants for keratin fibers is limited, a need for such dyes continues to exist.